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| Titel |
Estimating Tensile Rock Strength from InSAR Observations |
| VerfasserIn |
Sigurjón Jónsson, Zhong Lu |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054174
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| Zusammenfassung |
| The strength of rock is usually estimated in the laboratory using cm-scale cylindrical samples
of rock material that are free of fractures or other obvious defects. Bulk strength of rocks is
significantly lower than laboratory estimates, due to imperfections in the rock mass at scales
of meters and tens of meters. The problem is, however, that bulk rock strength is difficult to
determine, because large rock samples cannot practically be brought in for laboratory testing
and there are not many methods to determine large-scale rock strength. Here we show how
Interferometric Synthetic Aperture Radar (InSAR) measurements of a recent dyke intrusion
in western Saudi Arabia provide an unusual insight into tensional bulk strength of
rocks.
The dyke intrusion occurred in a lava province called Harrat Lunayyir in western Saudi
Arabia in April-July 2009 and caused a number of small to moderate-sized earthquakes along
with extensive surface faulting. The most intensive earthquake activity took place on
17-20 May when six magnitude 4.6-5.7 earthquakes occurred, resulting in some
structural damage and prompting the Saudi civil protection authorities to evacuate more
than 30000 people from the area. InSAR data of the area show that large-scale
(40 km à 40 km) east-west extension of over 1 m took place as well as broad
uplift amounting to over 40Â cm. The center of the uplifted area was transected by
northwest-trending graben subsidence of over 50Â cm, bounded by a single fault to the
southwest showing up to ~1Â m of normal faulting and by multiple smaller faults
and cracks to the northeast. The observed deformation is well explained with a
near-vertical dyke intrusion with a volume of ~0.1Â km3 and a NW-SE orientation,
approximately parallel to the Red Sea. The modeling suggests that the shallowest part of the
dyke reached within only 2Â km of the surface, resulting in the extensive surface
faulting.
We use ascending and descending Envisat interferograms and ascending ALOS data,
along with Envisat and ALOS multiple aperture radar interferometry (MAI) to determine the
3D displacements of the surface caused by the intruding dyke. From the 3D displacements we
generate strain maps that show a large amount of extension above the intrusion.
The interferometric data of this arid region are almost perfectly correlated, such
that lineations due to faulting and fractures can be accurately mapped by tracing
abrupt phase discontinuities. The discontinuity map and the 3D displacement map
confirm that up to 1Â m of normal faulting occurred to the southwest of the dyke on a
single fault, while in contrast, multiple fractures are seen to the northeast of the
dyke, exhibiting limited amount of vertical throw. Field observations affirm that the
discontinuities to the northeast are indeed tensional cracks with no significant vertical
displacement.
Comparison of observed tensional strain with the occurrence of tensional fractures
shows where tension exceeded the tensional strength limit of the surface rocks.
Surface strain exceeded 0.2Â millistrain in places, although this high amount of
surface strain is clearly coincident with tensional fractures, implying that the surface
bulk rock strength was exceeded. Assuming a shear modulus of 10Â GPa of the
surface rocks in the area, preliminary results show that the tensional bulk strength
of the rock is close to 1Â MPa, a value that is 5-10% of the strength that typically
is estimated from small cm-scale intact rock samples of similar rock types in the
laboratory. |
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